Psychological. Kindly advise me plz!

Hello Good Evening This problem belongs to my sister she's in relationship with another person but that person is very bad and rude person but when i asked to her she's saying I'll change him even me also can't finalized he's good or not recently my sister called with one another person like her brother and next she met her boyfriend he checked her mobile and saw the calls data even she explained he not listen he beat her than i involved solved but since that day iam very angry and next still repeated with small discussion finally recently he said if you not pick my call your parents will die after that very big fight done and he blaimed me that iam saying that to break up with him after that my sister also loose the confidence and after 2days hes called and said that I'll die i can't leave with out you plz he's black mailing my sister with photos and she give one chance to him for changing What should i do for this i want to change her psychological feelings kindly advise me plz